The present invention relates to an exhaust gas suppressor wherein secondary air heated inside the exhaust manifold of an internal combustion engine by the heat of exhaust gases from the engine is fed to the suction manifold downstream of the carburetor so as to prevent a mixture of primary air and fuel to be sucked into the combustion chamber from becoming too rich and assist in complete combustion of said mixture, thereby suppressing the generation of harmful gases contained in exhaust gases.
There have been various devices for suppressing the generation of harmful gases, and these devices may be classified into two types, namely, wet type and dry type. In the wet type, water or steam is injected into the suction manifold to lower the mixture explosion temperature by the heat of gasification of said water or steam, thereby suppressing the generation of nitrogen oxides (NO.sub.x). This type, however, requires a tank for storing water and if such tank is relatively small, frequent supply of water to the tank must be made, while if it is large, a large space for the installation of the tank is required, thus limiting the application of the device. Moreover, when the engine temperature is low, especially at the start of engine operation, part of the water or steam injected into the suction manifold forms drops when injected into the combustion chamber, so that the distribution of air-fuel mixture in the case of a multicylinder engine is degraded. Further, such drops of water detract from the smoothness of engine operation during transition as when the engine is accelerating or decelerating. The wet type device should theoretically be worth noticing, but is is hardly practicable since it has many problems in practice, as described above.
On the other hand, the dry type device includes the so-called air injection type wherein secondary air which is not superheated is fed to the suction manifold so as to prevent the fuel which remains in the suction manifold from making too rich the air-fuel mixture to be fed to the combustion chamber. Thus, the dry type is intended solely to reduce the air-fuel ratio of the primary air-fuel mixture (to, e.g. about 18:1) by feeding secondary air to the suction manifold. Thus, it places importance on the air-fuel ratio and is capable of decreasing CO contained in exhaust gases but incapable of decreasing the NO.sub.x and HC contents of exhaust gases. Further, classified as the dry type is a suction manifold heating device which does not feed secondary air to the suction manifold but which instead externally heats the suction manifold itself to heat the air-fuel mixture therein, thereby assisting in complete combustion of the mixture so as to suppress the generation of harmful gases contained in exhaust gases. However, since this type of device is intended to evaporate the drops of fuel in the air-fuel mixture by heating the suction manifold, such heating requires so much heat as to make it insufficient to make use of the exhaust gases and cooling hot water as heat source. Therefore, the heater is necessarily large in size and complicated, besides of costing much.